Synthesis and transport properties of type II clathrates

We present electrical resistivity and thermal conductivity for low alkali content NaSi₃₆, and compare it to the "fully loaded" Cs₈Na₁₆Si₁₃₆. Our results show that the electrical resistivities of the high and low alkali content materials differ greatly. Na₁Si₁₃₆ possesses a low thermal conductivity, similar in magnitude and temperature dependence to that of the "empty" clathrate Si₁₃₆. Extensive attempts at synthesizing Na_xGe₁₃₆ did not produce the clathrate phase in high enough yield for transport measurements, but instead an initially unidentified Na-Ge phase was prevalent. In order to clarify confusion in the literature, we propose a structural model for this novel phase, with resulting composition Na_1-xGe₃. The potential of type II clathrates for thermoelectric applications is reviewed.